Homeostasis and Stimuli in Living Organisms

Homeostasis and Environmental Response

All living things interact with their environment, including their surroundings and other organisms.

The environment is constantly changing. To survive and reproduce, living organisms must detect these changes and respond appropriately.

Responding to Stimuli

1. A stimulus is detected by receptors (structures or organs).
2. The stimulus is relayed to coordination systems, which interpret the signal and produce a response.
3. The organism responds using effectors (organs or structures).

Maintaining Internal Balance

To survive, living organisms must regulate their internal conditions.

Examples of internal conditions include temperature, water content, and concentrations of mineral salts and glucose in the blood.

Changes in the external environment and an organism's reactions can affect internal conditions.

The ability of an organism's body systems to maintain stable internal conditions is called homeostasis.

Types of Stimuli

Stimuli are detectable changes in an organism's internal or external environment.

Physical Stimuli

• Light: What we see
• Thermal: Temperature
• Mechanical: Vibrations and sounds
• Electrical: E.g., sharks detecting fish under sand
• Magnetic: E.g., birds navigating using Earth's magnetic field
• Gravitational: Gravity

Chemical Stimuli

• Chemical: What we smell and taste

Types of Receptors

Receptors are structures that detect stimuli.

• Interceptors: Collect information about the body's internal conditions; mostly respond to chemical stimuli.
• Exteroceptors: Provide information about the external environment; located on the body surface. Sense organs are exteroceptors.
• Proprioceptors: Provide information about the body's position, the position of its parts, and whether it is moving.

In unicellular organisms, receptors are molecules or cell organelles. In plants, receptors are cells that don't form specialized organs.